The invention relates to a particle detector. The invention also relates to a method of detecting particles.
Each year millions of freight containers are unloaded at almost 400 U.S. seaports. Additionally, there are nearly 1 million public and private airplane flights landing on U.S. soil and 11 M trucks and 2M rail cars that enter the U.S. from Canada and Mexico each year. Also, there are about 50 vehicle border crossings to the U.S. with typically 5-10 traffic lanes. Particle detectors such as, for example, neutron detectors are currently used at seaports, airports, rail yards, and border crossings to scan for contraband special nuclear material (SNM) and to prevent terrorists from smuggling said material, e.g., a fission bomb or fissionable material, into the U.S. Fissioning nuclei generally emit neutrons of many different energies but most of these neutrons rapidly lose energy and reach kinetic energies typical of the surrounding material. These “thermal” neutrons have kinetic energies approximately equal to a kBT of ˜25 MeV where kB is the Boltzmann constant and T is room temperature. Detection of thermal neutrons is typically the most important. Scanning each freight container, airplane, truck, rail car and vehicle that enters the U.S. would require thousands of neutron detectors.
One of the most common large-area detectors for thermal neutrons employs He-3 isotopically enriched gas in a cylindrical tube proportional counter. Alternatively, a multiwire proportional counter (MWPC) may be used with He-3. However, there is a serious shortage of the He-3 isotope while the demand continues to rise for detectors of SNM at U.S. ports and in foreign locations. Except for gaseous He-3, which has a very large thermal neutron cross section and can also serve as the main component of a proportional counter, most neutron detectors need a separate neutron activation layer which includes a material that has a high concentration of an isotope with a large neutron cross-section. Such isotopes include He-3, Li-6, B-10, and Cd-113. These isotopes not only capture slow or thermal neutrons very well, but then emit high energy charged particles that are easier to detect.
One type of nuclear particle detector is based on gas discharge proportional counters or those operating in the Geiger discharge region and called Geiger tubes. For large-area detectors, these may take the form of a multiwire proportional detector or may use an array of tubular structures. If they are not filled with He-3 gas, they typically need a neutron activation material in addition to the gas for the proportional detector. These proportional counters typically operate at 1000 V or higher, are bulky, and are sensitive to pressure and temperature changes. Another common detector type is designed around a scintillator material, either solid or liquid, that emits bursts of light when a charged particle passes through it. This light is then detected by a photomultiplier or other photon detector. The scintillator light is usually emitted in the near UV or visible range of wavelengths. Scintillator detectors are also typically bulky, complex and can be very sensitive to gamma-ray background counts.
Yet another type of detector is based on a high purity, single crystal semiconductor together with one of the activating isotopes. In this case, the high-energy charged particles produced by the neutron activator material will directly generate a large number of electron-hole pairs in the semiconductor as the charged particle passes through the semiconductor. These electrons and holes are collected in the same semiconductor, usually by applying a strong external voltage to create an electric field in the semiconductor which separates the electrons and holes and sweeps them to the positive and negative electrodes. Unlike the scintillator crystal, particle detection occurs entirely in one detector structure. In most cases, the semiconductor material must be from 1 to 10 mm thick.
One frequent limitation of the semiconductor detector is that usually high-quality single crystals are needed in order to obtain a high probability of collection of the generated charge carriers. Due to the limited size available for most high quality single crystals, achieving large-area detection from single-crystal materials then requires assembling an array of smaller crystals or using an array of thinly sliced wafers of single crystals which significantly increases the costs. Additionally, effective use of the neutron activation material and the semiconductor often requires that the semiconductor be microstructured with pores or grooves to accommodate the neutron activator layers. A further limitation of this type of neutron detector is its sensitivity to radiation damage from radiation that pervades many environments in which the detector is to be used which degrades the performance of the detector over time. In addition, the large semiconductor volume leads to sensitivity to gamma-ray background noise.
Furthermore, the aforementioned particle detectors are typically bulky and difficult to use. If the footprint for and portability of a particle detector was improved, security at seaports, airports, rail yards, and border crossings would increase. Additionally, if such improvements to particle detectors were made, demand for the detectors in other applications and industries would increase. For example, demand for a particle detector with the capability to detect neutrons utilized in down-hole well-logging applications in the oil and gas industry would increase if the aforementioned improvements were made.
Thus, it would be desirable to provide a particle detector that addresses the deficiencies described above. It would also be desirable to provide a method of detecting energized charged particles which addresses the aforementioned deficiencies.